1. Field of the Invention
The present invention relates to a curve approach control apparatus which controls a vehicle to make it travel a curved road at an appropriate speed.
2. Description of the Related Art
Many curve approach control apparatus have been proposed which use road map data in a navigation device to detect an overspeed state of a vehicle with respect to a curve in front and activate an alarm or deceleration control. For example, Japanese Patent Unexamined Publication No. Hei. 4-236699 (JP-A-4-236699) discloses a technology which determines an appropriate approach speed at which to enter the curve based on a radius of curvature of a curve in an optimum travel path (guided path) set in the navigation device and on detected road surface conditions and, when an actual vehicle(car) speed is higher than the calculated value of the appropriate approach speed, issues a warning to alert the driver to the need for reducing the vehicle speed, or automatically activates a vehicle speed reduction means in combination with the warning.
A method of determining a curve's radius of curvature by using road map data in the navigation device is disclosed, for example, in Japanese Patent Unexamined Publication No. Hei. 11-2528(JP-A-11-2528) filed by the applicant of this invention which describes a technology for determining the curve's radius of curvature from nodes on the road map data of the navigation device.
As shown in FIGS. 10A and 10B, it is often the case that in expressways or wide highways (hereinafter referred to as "main roads") there is a large space at a branch point or merge point leading to an entrance road or exit road to and from the main road. Hence, when a driver moves into or out of the main road, he or she generally utilizes the wide space near the branch or merge point to perform as small a cornering operation as possible.
Nodes on a road map in a navigation device are simply plotted near the center of the road, so that a locus of the nodes plotted on the entrance/exit road to/from the main road apparently represents a significantly tighter curve than the actual locus of the vehicle moving into or out of the main road.
Hence, when the vehicle(car) moves into or out of the main road, if a radius of curvature of the curve is determined based on the node data supplied from the navigation device and, based on the curve's radius of curvature, the alarm control and the deceleration control are performed, these controls may become excessive because they are intended for an apparently sharp curve.